1. Field of the Invention
The present invention relates to the fabrication of semiconductor structures, and more particularly, to a method for forming selective protection layers over copper interconnects in the manufacture of integrated circuits.
2. Description of the Related Arts
As integrated circuit feature sizes continue to decrease, it has become advantageous to construct metal connections out of copper instead of aluminum. Copper has a lower resistivity than aluminum, and therefore can form higher speed connections for a given line width.
Copper has disadvantages when compared to aluminum that must be overcome. For example, copper is much more susceptible to oxidation during processing. Copper also tends to diffuse into adjacent materials, including dielectrics. To use copper for interconnections, therefore, it is necessary to encapsulate the copper in barrier materials.
Where a low resistive interface between a copper interconnect and an underlying metal conductor must be completed, it is common in the art to deposit a barrier of a metal material, such as titanium, tungsten, tantalum, or tantalum nitride before the copper layer is deposited.
After the copper layer is deposited, another barrier layer, typically called a sealing layer is deposited overlying the copper. Typically in the art, this sealing layer (also called a cap layer, or an encapsulation layer) is composed of silicon nitride, though other materials are used.
Referring to FIG. 1, a cross-section of a prior art damascene wiring is shown. A dielectric layer 102 and a substrate 100 are depicted. The substrate 100 encompasses all underlying layers, devices, junctions, and other features that have been formed prior to the deposition of dielectric layer 102.
Interconnection trenches are formed in the dielectric layer 102, and a barrier layer 104 is deposited overlying the dielectric layer 102 and in the trenches. The purpose of the barrier layer 104 is to prevent diffusion of the subsequently deposited copper. A copper layer 106 has been deposited overlying the dielectric layer 102 and filling the trenches. The excess copper layer 106 and the excess barrier layer 104 are then typically polished down to the dielectric layer 102 by a chemical mechanical polishing (CMP) process.
Following the definition of the copper interconnects 106 by the CMP, a sealing layer 108 is deposited. This sealing layer 108 serves as a barrier to diffusion of the copper layer 106 into any overlying layers. In addition, by encapsulating the copper completely in the barrier layer 104 and the sealing layer 108, oxidation during subsequent processes is eliminated.
The sealing layer 108 is typically silicon nitride. Due to the need for low temperature processing after the copper is deposited, the silicon nitride layer 108 cannot be deposited at temperatures in excess of 450xc2x0 C. Accordingly, silicon-nitride deposition is typically performed using plasma enhanced chemical vapor deposition (PECVD) where temperatures generally range from 200xc2x0 C. to 425xc2x0 C. PECVD silicon-nitride has been used in other applications in semiconductor devices. However, in using a silicon nitride cap for copper interconnects, conventional PECVD silicon nitride creates reliability problems. In particular, silicon nitride films deposited using conventional PECVD processes have poor adhesion to copper surfaces. As an example, some nitride films can be peeled from the copper surfaces simply by scratching the film or by removing the film using an adhesive tape. These results are indicative of how the silicon nitride film might adhere to the copper in an actual fabrication process. After being deposited onto the copper surface, additional insulating layers will be deposited over the silicon nitride film. However, subsequent deposition of insulating layers onto the nitride film will produce stresses which can cause the silicon nitride layer to peel from the copper surface. Despite that other layers have been deposited onto the semiconductor device, the peeling of the silicon nitride film creates a path for copper to diffuse outward and for moisture or other contaminates to diffuse inward.
Several prior art approaches deal with copper sealing methods. U.S. Pat. No. 5,403,779 to Joshi et al teaches a process to form damascene metal lines and vias. After copper deposition, tungsten is deposited by CVD. A CMP step is performed to planarize the structures. U.S. Pat. No. 5,447,887 to Filipiak et al discloses a process to form copper interconnects where a copper silicide layer is formed overlying the copper traces. A silicon nitride layer is deposited overlying the copper silicide to complete the encapsulation layer. U.S. Pat. No. 5,814,557 to Venkatraman et al discloses a process to form a copper-containing aluminum film overlying an aluminum conductor. An annealing operation is performed to diffuse the copper into the aluminum conductor.
An object of the invention is to provide a process for implementing copper damascene wiring within an integrated circuit.
A further object of the invention is to provide a method of fabricating copper interconnects with selective protection layers that are selectively formed only overlying the copper interconnects.
A still further object of the invention is to provide a method of fabricating copper interconnects with selective sealing layers that effectively prevent copper oxidation and diffusion.
A yet further object of the invention is to provide a method of fabricating copper interconnects with selective adhesion layers that increase the adhesion between the nitride and copper.
The above and other objects are achieved by polishing the excess copper layer with a slurry comprising an aluminum organic substance. The aluminum organic substance reacts with copper via annealing to selectively form aluminum-copper alloys on the copper interconnects. The aluminum-copper alloys are then oxidized to form aluminum oxide protection layers capping the copper interconnects. The aluminum oxide protection layers can function as a conventional sealing layer to prevent copper diffusion and oxidation, or they can function as an adhesion layer to improve the adhesion between the copper interconnects and a later sealing layer.
According to one aspect of this invention, there is provided a method for making copper interconnects in the fabrication of a semiconductor device comprising the steps of providing a semiconductor substrate; forming a dielectric layer overlying the substrate, the dielectric layer having interconnect trenches formed therein; depositing a copper layer overlying the dielectric layer and filling the interconnect trenches; chemical-mechanical polishing the copper layer to the top surface of the dielectric layer and thereby defining copper interconnects, said polishing leaving an aluminum organic substance on said copper interconnects which can react with copper via annealing to form an aluminum-copper alloy; annealing the substrate to form aluminum-copper alloys overlying said copper interconnects; and oxidizing the aluminum-copper alloys to form aluminum oxide protection layers capping said copper interconnects.
According to another aspect of this invention, there is provided a method for making copper interconnects in the fabrication of a semiconductor device comprising the steps of providing a semiconductor substrate; forming a dielectric layer overlying the substrate, the dielectric layer having interconnect trenches formed therein; depositing a copper layer overlying the dielectric layer and filling the interconnect trenches; first chemical-mechanical polishing the copper layer with a first slurry to remove most of the copper layer overlying the dielectric layer; second chemical-mechanical polishing the copper layer with a second slurry to the top surface of the dielectric layer and thereby defining copper interconnects, said second slurry comprising an aluminum organic substance which can react with copper via annealing to form an aluminum-copper alloy, and said polishing leaving said aluminum organic substance on said dielectric layer and said copper interconnects; annealing the substrate to form aluminum-copper alloys overlying said copper interconnects; and treating the substrate with an inorganic acid solution to selectively remove non-reacted aluminum organic substance from said dielectric layer and to oxidize the aluminum-copper alloys to form aluminum oxide protection layers capping said copper interconnects.
According to still another aspect of this invention, there is provided a method for making copper interconnects in the fabrication of a semiconductor device comprising the steps of providing a semiconductor substrate; forming a dielectric layer overlying the substrate, the dielectric layer having interconnect trenches formed therein; depositing a barrier layer lining said interconnect trenches and overlying said dielectric layer; depositing a copper layer overlying the barrier layer and filling the interconnect trenches; first chemical-mechanical polishing the copper layer with a first slurry to remove most of the copper layer overlying the dielectric layer; second chemical-mechanical polishing the copper layer and the barrier layer with a second slurry to the top surface of the dielectric layer and thereby defining copper interconnects, said second slurry comprising an aluminum organic substance which can react with copper via annealing to form an aluminum-copper alloy, and said polishing leaving said aluminum organic substance on said dielectric layer and said copper interconnects; baking the substrate at a first temperature; annealing the substrate at a second temperature higher than the first temperature to form aluminum-copper alloys capping said copper interconnects; and treating the substrate with an inorganic acid solution to selectively remove non-reacted aluminum organic substance from said dielectric layer and to oxidize the aluminum-copper alloys to form aluminum oxide cap layers overlying said copper interconnects.